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ABSTRACT, NaCO 3 -, NaSO 4 -, CaOH + , MgOH + . Calculation method is proposed for quantitative assessment of real ion forms in the soil solution of chestnut solonetz soil complex. Were proposed equations to calculate free and associated forms of ions. To solve the equations were used an iteration, a linear interpolation of equilibrium constants, a Method of Ionic Pairs including a law of initial concentration preservation, a law of the operating masses of equilibrium system, the concentration constants of ion pair dissociation on the law of operating masses. Was determined the quantity of ion free form and a coefficient of ion association as ratio of ions free form to analytical content γ e = C ass /C an . The association of ions varies in individual soils and soil layer. Increasing soil solution salinity amplifies the ions association. In form of ionic pairs in soil solution are: 11.8-53.8% of Ca 2+ ; 9.4-57.3% of Mg 2+ ; 0.7-11.9% of Na + ; 2.2-22.3% of HCO 3 -, 11.8-62.7% of SO 4 2-. The ion CO 3 2-is high associated, the share of ions in associated form is up to 92.7%. The degree of soil solution saturation was obtained for three level of approximation accounting on analytical concentration, calculated association coefficient, calculated coefficient of association. Relating to thermodynamic solubility product S 0 , the mathematical product of analytical ionic pairs indicated super saturation of soil solutions up to K 1 = 100, taking into account calculated coefficient of association ion activity super saturation of soil solutions is absent, K 3 ≈1. Only for solonetz chestnut meadow K 3 ≈2-5. The soil solution saturation degree in soil profile and laterally in landscape varies. The quantitative assessment of real ion forms in the soil solution allows explain evolution of landscape of salted soils, structure of soil cover. Calculations fulfilled show that a possibility of soil degradation scenario taking into account the laws of association of ions in soil solution is much more probable and dangerous than it was assessed before. New understanding of water-salt transfer, geochemical barriers and ecological functions of soil will help to improve rainfed and irrigational agriculture.

Abstract. The assessment of soil and vadose zone as the drains for carbon sink, proper modelling of the effects and extremes of biogeochemical cycles in terrestrial biosphere are the key components to understand the carbon cycle, global climate system, aquatic and terrestrial systems uncertainties. Carbonate-calcium equilibrium causes saturation of solution with CaCO3, determines its material composition, migration and accumulation of salts. In solution are formed electrically neutral ion pairs СаСО3°, CaSO4°, MgCO3°, MgSO4°. Charged ion pairs CaHCO3+, MgHCO3+, NaCO3−, NaSO4−, CaOH+, MgOH+ are developed the carbonate-calcium equilibrium algorithm, mathematical model and original software to calculate the real equilibrium forms of ions and determine the nature of carbonate-calcium balance in solution. An approach conducts the quantitative assessment of real ion forms of solution in solonetz soil and vadose zone of dry steppe taking into account the ions association at high ionic strength of saline soil solution. The concentration of free and associated ion form were calculated according to analytical ion concentration in real solution. In iteration procedure were used the equations of ion material balance, a linear interpolation of equilibrium constants, a method of ionic pairs, the laws of initial concentration preservation, operating masses of equilibrium system, the concentration constants of ion pair dissociation. The coefficient of ion association γe was determined as a ratio of ions free form to analytical content of ion γe=Cass/Can. Depending on soil and vadose zone layer, concentration and composition of solution in the ionic pair's form are: 11–52% Ca2+; 22.2–54.6% Mg2+; 1.1–10.5% Na+; 3.7–23.8 HCO3−, 23.3–61.6% SO42−, up to 85.7% CO32−. The carbonate system of soil and vadose zone water solution helps to explain the evolution of salted soils, vadose and saturation zones, and landscape, improve the soil maintenance, plant nutrition and irrigation. The association of ions in soil solutions is one of the drivers, promoting transformation of solution, excessive fluxes of carbon in the soil, loss of carbon from soil through vadose zone.

Carbonate system determines the basic processes of soil formation and evolution. The aim of research was to show the importance of chemical Carbonate Calcium Equilibrium (CCE) in soil solutions. CCE causes a degree of soil solution's saturation with CaCO 3 as a factor of carbonate geochemical cycle. CCE depends on chemical composition, pH, Eh, buffering properties of liquid phase, dissolution, migration, precipitation of carbonates in the soil profile and landscape, ion exchange processes at the interface of soil solid and liquid phases. At high ionic force in soil solution are formed electrically neutral ion pairs СаСО 3 0 ; CaSO 4 0 , MgCO 3 0 , MgSO 4 0 and charged ion pairs CaHCO 3